Disable MessageLoop features on some BrowserThreads.

base/message_loop/message_loop.h

Index: base/message_loop/message_loop.h
diff --git a/base/message_loop/message_loop.h b/base/message_loop/message_loop.h
index 4f3250c3b7f1b164d984dc0e4dcee28866092015..1b8351ba9498675e6e9d2e2120d5dda4cfb02cb0 100644
--- a/base/message_loop/message_loop.h
+++ b/base/message_loop/message_loop.h
@@ -316,6 +316,15 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
   // Runs the specified PendingTask.
   void RunTask(const PendingTask& pending_task);
 
+  // Disables running nested loops and Add/RemoveNestingObserver().
+  void DisableNesting() { enable_nesting_ = false; }
+
+  // Disables Add/RemoveDestructionObserver().
+  void DisableDestructionObservers() { enable_destruction_observers_ = false; }
+
+  // Disables Add/RemoveTaskObserver().
+  void DisableTaskObservers() { enable_task_observers_ = false; }
+
   //----------------------------------------------------------------------------
  protected:
   std::unique_ptr<MessagePump> pump_;
@@ -451,6 +460,15 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
   // MessageLoop is bound to its thread and constant forever after.
   PlatformThreadId thread_id_;
 
+  // Enable running nested loops and Add/RemoveNestingObserver().
+  bool enable_nesting_ = true;
+
+  // Enable Add/RemoveDestructionObserver().
+  bool enable_destruction_observers_ = true;
+
+  // Enable Add/RemoveTaskObserver().
+  bool enable_task_observers_ = true;
+
   DISALLOW_COPY_AND_ASSIGN(MessageLoop);
 };